Closed-loop CNC machine system and method

ABSTRACT

A closed-loop feedback system and method for performing a machining operation. A tool&#39;s operational properties are measured in real-time. The operating parameters of the machining process are adjusted in real-time based on the measured properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.11/450,444 filed on Jun. 12, 2006.

BACKGROUND OF THE INVENTION

CNC or Computer Numerical Control has been used in machining operationsfor decades to automate machining operations that previously wereperformed by a manual operator. Such operations include moving a tool orworkpiece in relation to each other in three dimensional space in orderto perform an operation on the workpiece. CNC programming consists ofgenerating computer commands that are passed to a machine tool that hasa CNC control. The commands instruct the control on what tool paths themachine tool should take and sets various machining conditions such asthe feed, or speed the tool cuts into the workpiece, and spindle speed,or the speed with which the tool rotates when cutting the workpiece.

A CNC machine is a computer-controlled machine, which employs a simplesoftware language to control a mechanically complex machine using open-and closed-loop control mechanisms. CNC machining is divided into twomain types of machining processes: milling and turning. Milling isperformed by a milling machine, and involves positioning a non-rotatingwork-piece underneath a spindle, which removes material using a sharp,rotating cutter, called an end mill. Turning is performed by a lathe,and involves spinning a round work-piece in a machine spindle andcutting the work-piece using a non-rotating tool bit. A picture of atypical CNC Lathe is shown in FIG. 1.

The primary goals of any CNC turning operation are quality and speed.Five main factors influence the quality and speed of a turningoperation: 1) Rigidity of cutting tool, 2) Sharpness of cutting tool, 3)Hardness of work-piece, 4) Speed of work-piece rotation (“spindlespeed”), and 5) Amount of material removed (“feed rate”).

Turning operations are divided into two basic types: inside turning(also called “boring”) and outside turning. Outside turning can beperformed using a very rigid cutter, because the size of the tool is notrestricted by the diameter of the workpiece. A picture of outsideturning is shown in FIG. 2. Inside turning, on the other hand, restrictstool geometry, and therefore stiffness, because the cutter must be ableto fit inside the work-piece. A picture of inside turning is shown inFIG. 3.

Inside turning can be challenging to optimize, because the five mainfactors listed above are inter-related and achieving a productivebalance often involves trial and error. A machinist typically approachesan inside turning operation with 3 out of the 5 factors defined inadvance. Sharpness of cutting tool, rigidity of cutting tool, andhardness of work-piece material are typically defined during the initialsetup of a turning operation. The machinist optimizes the remaining 2factors, speed of cut and feed rate of cut, by changing softwareparameters in the CNC Program. This optimization is often the result oftrial and error: the machinist loads a work-piece into the machine andcommands the machine to execute the CNC program. The machinist thenremoves the work-piece and examines its quality. If the quality isunsatisfactory, the machinist will make one or more changes to the CNCprogram, and repeat the process until a quality part is produced.

CNC machines are a combination of mechanical hardware, computer controlsystems, and computer software. CNC machines are mechanically complex,and the variety and robustness of product offerings represent some ofthe best American mechanical engineering. Evolution of mechanical designhas continued since the industrial revolution. For the most part, thecontrol systems and software engineering are relatively simplistic bycomparison. While software engineering and computers have grownexponentially in consumer and scientific markets, industrial controlsfor manufacturing have fallen behind and still employ the same basicalgorithms developed in the 1950's and 60's. Accordingly there remains aneed drive the state-of-the-art if the United States is to remaincompetitive in the world market for industrial manufacturing automation.There further remains a need for the evolution of closed-loop controlsystems for manufacturing automation for improved productivity. Thepresent invention fills this need by providing a method and system thatprovides real-time control of the machining operation by measuringproperties of the tool in real-time and adjusting the machiningparameters in real-time.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a method for performing amachining operation on a workpiece. The method includes providing a toolfor physically removing material from the workpiece. An algorithm isprovided setting the initial operating parameters for the machiningoperation. At least one physical property of the tool is measured inreal-time. The algorithm is adjusted in real-time based on the at leastone measured physical property.

Another embodiment of the present invention is a closed-loop feed backCNC controlled inside turning process performed on a workpiece. Themethod includes providing a boring bar. An initial CNC code is providedthat sets the initial spindle speed and feed rate for the turningprocess. At least one of the force on the boring bar, the deflection ofthe boring bar, and the vibration of the boring bar is measured inreal-time during the inside turning process. The CNC code is adjusted inreal-time to set a new spindle speed and feed rate based on thereal-time measurement.

Another embodiment of the present invention is a closed-loop feedbackcontrol machining system. The system includes a machine, a tool, atleast one sensor, at least one signal analyzer, and at least onecontroller. The machine includes a workpiece holder, a tool holder, aspindle drive system and a feed drive system. The spindle drive systemprovides relative rotation between a workpiece and the tool. The feeddrive system provides relative translational movement between aworkpiece and the tool. The at least one sensor is operativelyassociated with the tool. The at least one signal converter isoperatively associated with the at least one sensor. The at least onesignal analyzer is operatively associated with the at least one signalconverter. The at least one controller is operatively associated withthe at least one signal analyzer and operatively associated with thespindle drive system and the feed drive system. The at least one sensorreceives a signal from the tool and transmits the signal to the at leastone signal converter. The at least one signal converter converts thereceived signal to a set of digital parameters and transmits the digitalparameters to the at least one signal analyzer. The at least one signalanalyzer determines in real-time at least one of the force on the tool,the deflection of the tool, and the vibration of the tool. The at leastone controller adjusts the power exerted by at least one of the spindledrive system and the feed drive system in real-time based on thereal-time determinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a picture of a typical CNC lathe.

FIG. 2 shows a picture of a typical outside turning process.

FIG. 3 shows a picture of a typical inside turning process.

FIG. 4 shows a block diagram of a typical CNC machine.

FIG. 5 shows a picture of a typical boring bar performing an insideturning process.

FIG. 6 shows a block diagram of a closed-loop feedback CNC controlledmachining operation of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For simplicity and illustrative purposes, the principles of the presentinvention are described by referring to various exemplary embodimentsthereof. Although the preferred embodiments of the invention areparticularly disclosed herein, one of ordinary skill in the art willreadily recognize that the same principles are equally applicable to,and can be implicated in other compositions and methods, and that anysuch variation would be within such modifications that do not part fromthe scope of the present invention. Before explaining the disclosedembodiments of the present invention in detail, it is to be understoodthat the invention is not limited in its application to the details ofany particular embodiment shown, since of course the invention iscapable of other embodiments. The terminology used herein is for thepurpose of description and not of limitation. Further, although certainmethods are described with reference to certain steps that are presentedherein in certain order, in many instances, these steps may be performedin any order as may be appreciated by one skilled in the art, and themethods are not limited to the particular arrangement of steps disclosedherein. Further, although certain embodiments are shown in the figures,the present invention is certainly not intended to be limited to theseportrayed embodiments.

CNC machines utilize a combination of open-loop and closed-loop controlsystems. A closed-loop control system consists of an output coupled toan input via a control algorithm, and, it provides the best accuracysince output is compared to input and appropriate compensation is addedto make the two match. A block diagram for a typical CNC machine isshown in FIG. 4. Physical measurements of dimensions such as tool lengthand part diameter are examples of closed-loop control in typical machinetools. These machines are capable of producing a part, measuring thephysical dimensions of the part, and making parameter adjustments tocorrect inaccuracies. However, typically a human operator is responsiblefor making adjustments to CNC machine code to account for poor surfacefinish and/or accelerated tool wear. The human operator is likely tomake a single optimization, during the initial setup of a part routine.This optimization will attempt to account for tool wear throughout theproduction run, however it will be a one-time compromise with minimalreal-time compensation for change. Typical CNC equipment has noprovision for autonomously correcting qualitative parameters, such aspoor surface finish caused by tool vibration (or “chatter”). In order tomake strides in closed-loop control systems, the CNC machine needsadditional process inputs.

As used herein, “machining operation” refers to any process where amachine holds a tool and the tool contacts a workpiece. Typicalmachining operations include milling and turning. Typically, millingmachines are used for milling and lathes are used for turning. Eachmachine typically also includes a tool holder. As used herein, “tool”refers to the actual device that contacts a workpiece and not the entiremachine itself. Typical tools include end mills used in milling machinesand boring bars used in lathes. As used herein, “measuring at least onephysical property” refers to both measuring the absolute value of thephysical property and measuring the rate of change of the property withrespect to time.

One embodiment of the present invention is a closed-loop control methodfor performing a machining operation on a workpiece with a tool. Themethod comprises providing a tool for performing a machining operation.The tool and the workpiece are provided with initial operationalparameters. Physical properties of the tool are measured in real-time.Based on these measurements, the operational parameters are adjusted tooptimize the machining operation.

Preferably, the machining operation is an inside turning process.Preferably the turning process is performed by a lathe and the tool is aboring bar. In this embodiment, the tool is stationary and the workpieceis rotated. The workpiece is also moved towards the tool in a directionthat is parallel to the axis of rotation of the tool. The workpiece isturned at a speed known as the spindle speed. The workpiece is providedwith an initial spindle speed. The workpiece is also movedtranslationally with respect to the tool at a speed known as the feedrate. The workpiece is provided with an initial feed rate. FIG. 5 showsa picture of a boring bar performing an inside turning process.

The measured tool properties may include any properties that can be usedto determine the quality of the machining operation in real-time.Preferably, the measured properties include the deflection of the tool,the forces exerted on the tool and the vibrations of the tool. Thedeflection, forces and vibrations of the tool while performing themachining operation on the workpiece may be determined by the use of atleast one sensor. Any sensor may be used that is rugged enough towithstand the harshness of the machining environment and provideaccurate measurement of the deflection, forces and vibrations. Exemplarysensors include magnetic sensors, analog strain sensors and fiber opticsensors. Analog strain sensors will provide measurement of the actualdeflection, force or vibration while magnetic and fiber optic sensorswill provide a measurement of the change of the property with respect totime. The actual value of the property is not a necessity because themeasurement is taken to predict a desirable change in the operatingparameters of machining process to improve part quality. The sensor maybe located in the tool itself, on the tool holder, or somewhere insidethe machine.

In the embodiment using fiber optic sensors, preferably at least onesensor is embedded in the tool. The sensor may be embedded in any partof tool. Preferably, the sensor is embedded in the far most end from thetool holder such that the greatest deflection, force and vibration canbe measured. In this embodiment, a light may be shined on the sensor inthe tool. The embedded sensor may return the light back to an electroniccontrol interface which converts the optical signal into digitalparameters that can be read by a signal analyzer. The interface may thensend the converted signal to a signal analyzer. The signal analyzer maythen analyze the signal and determine the deflection, forces andvibrations on the tool. This determination is based on the change inthese parameters causing a change in the optical signal received by theexternal sensor. The signal analyzer may then feed this information tomicroprocessor instituting an Active Tool Control Electronics FeedbackLoop. The Feedback Loop may then determine if the spindle speed or feedrate should be adjusted based on the determined tool properties. TheFeedback Loop may then create new algorithms for the spindle speed andfeed rate. The Feedback Loop may then provide these new algorithms to amachine controller, preferably a CNC machine controller. The controllermay then adjust the spindle speed and/or feed rate of the workpiece.This process may be performed continuously during the entire duration ofthe machining operation.

FIG. 6 shows a basic block diagram of the method of the presentinvention used with a CNC machine tool. A sensor senses physicalproperties of a tool or cutter that performs an operation on aworkpiece. The sensor communicates with an Active Tool ControlElectronics Feedback Loop. The Feedback Loop analyzes the sensedphysical properties and determines new operational parameters for theprocess based on the analysis. The Feedback Loop communicates withmotion control electronics such as amplifiers and servos. The amplifiersand servos change the operational parameters of the machining processbetween the cutter and workpiece. The Feedback Loop also communicateswith a machine control computer. The machine control computer includes auser interface and program storage. The user interface displays themeasured physical properties of the cutter and the changes made in theoperational parameters. The program storage stores this information forfuture reference. The machine control computer also communicates with aportable display device such as a cellular phone, PDA, or tablet PC.This provides remote communication of the information displayed on theuser interface to a remote machine operator. All of the steps shown inFIG. 6 are performed in real-time during the machining operation.

In the embodiment using a fiber optic sensor, a light source throwslight to a type of light manager that can both sense and transmit lightsignals. The light manager then throws a light signal to a fiber opticsensor embedded inside the tool being used in the machining operation.The sensor then throws a light signal back to the light manager. Thissignal will vary depending on the forces on the tool, the deflection onthe tool and the vibration of the tool. The light manager then throwsthe light signal to a second light manager. The second light managerreceives the signal and throws an appropriate signal to both a tunablefilter and a high speed detector. The filter filters the light and sendsan appropriate signal to the high speed detector. The detector thendetects the signal and converts it to a signal that can be read by asignal analyzer. The detector then sends the converted signal to thesignal analyzer. The signal analyzer may then analyze the signal anddetermine the values of the physical properties of the tool. The signalanalyzer may then provide these values to an Active Tool ControlElectronics Feedback Loop and the Feedback Loop uses the values asdescribed above. This process may be performed continuously during theentire duration of the machining operation. The above description is notmeant to be a comprehensive description of the operation of a fiberoptic sensor. Those of ordinary skill in the art understand theoperation of such a sensor and therefore this detailed description isomitted. Bartow et al. in “Fiber Bragg grating sensors for dynamicmachining applications” Proceedings of the SPIE, Volume 5278, pp. 21-31(2003), which is hereby incorporated by reference in its entirety,describe use of a fiber optic sensor embedded in a tool to measurevibrations in the tool.

In the embodiment using magnetic sensors, a sensing element may not needto be embedded in the tool. In this embodiment, a magnetic field may begenerated in the vicinity of the tool while the tool is in operation.The tool will then create its own magnetic field. The tool-createdmagnetic field will vary with the forces on the tool, the deflection ofthe tool and the vibration of the tool. A sensor may then receive thetool-created magnetic field and send it to a signal converter that mayconvert the signal to a signal that can be read by the signal analyzer.The signal analyzer could determine the deflection, forces and vibrationof the tool based on the change in the signal received. The processwould then continue as in the embodiment using the fiber optic sensor.

Of course any type of sensor or sensing devices can be used that hasruggedness to withstand harsh industrial environments and delivers therequired accuracy in terms of vibration and deflection measurements.Other possible sensors include analog electronic strain gauges. However,fiber optic sensors are preferred because optical measurement has betternoise immunity and better frequency response than traditional analogelectronic strain gauges.

In a preferred embodiment, at least one of the signal transfers occurswirelessly. In a more preferred embodiment, at least the signal from theembedded sensor to the signal converter and the signal from the signalconverter to the signal analyzer are communicated wirelessly. This wouldallow machines to be retrofitted with closed-loop control methodology ofthe present invention in a simple and inexpensive manner.

The measured parameters can also be used to determine that a tool isworn and should be replaced. For example, quality and speed of themachining operation depend on tool properties including the tool'srigidity and sharpness. The less sharp the tool is the slower themachine operation will need to be performed to achieve the same quality.Thus, as the tool wears, the Active Tool Control Electronics FeedbackLoop would determine that the spindle speed and/or the feed rate shouldbe decreased. In one embodiment of the present invention, if theadjusted spindle speed or feed rate falls below a certain value amachine operator could be notified that the tool should be replaced. Thecontroller could also cause the machine to shut off if the speed or feedrate falls below a certain value.

The measurements of the tool properties, the adjustments made to themachining operation parameters, and tool wear alerts can be communicatedto a machine operator. Preferably, communication is performed wirelesslyto a portable display device such as cellular phone, a personal digitalassistant (PDA) or a tablet or laptop PC. Such methods of wirelesscommunication are well known in the art and detailed description isomitted here. The machine operator may choose to override thecontroller's adjustments.

Another embodiment of the present invention is a system for performing amachining operation. The system includes a machine, a tool, at least onesensor, an electronic control hardware interface; machine controlsoftware and a controller.

The machine may be any type of machine which uses a tool to perform anoperation on a workpiece. Exemplary machines include milling machinesand lathes. Preferably, the machine is a lathe. The tool may include anytool that is commonly used in machining. Preferably, the tool is aboring bar used for inside turning or boring. The at least one sensormay be any type of sensor capable of withstanding harsh operatingconditions and accurately measuring tool parameters such vibration,deflection and force, as described above. Exemplary sensors includeanalog electronic strain gauges, fiber optic sensors and magneticsensors. Preferably, the at least one sensor is a fiber optic sensor.The hardware interface is not limited to any particular design but mustbe capable of incorporating the input measurement from the sensor intothe controller. The machine control software must be capable ofincorporating the input measurement into the algorithm used by thecontroller. Preferably, the controller is a CNC device.

In one embodiment, the system may be retrofitted onto a machine with anexisting CNC controller. In this embodiment, the system may include arugged optical sensor embedded within a tool such as a boring bar, aminiature, rugged, control electronics suite, integrated into theexisting CNC control, and a software suite which incorporates feedbackfrom optical sensor into the CNC.

The present invention will improve the performance ofComputer-Numerically-Controlled (CNC) equipment by incorporating aclosed-loop control system to vary cut parameters such as speeds andfeeds based on input from a sensor such as a fiber-optic sensor whichmeasures tool forces, deflections and vibrations. The present inventionwill permit CNC equipment to perform at a higher level of productivity,managing an increased number of process parameters under autonomousmachine control. The increased autonomy will yield higher qualityproduct, and will free the machine operator to focus on tasks that arebetter suited to human technicians, such as overall systemoptimizations, and other qualitative improvements. The present inventionby providing closed-loop feedback to a manufacturing process at theequipment level will ensure production of a higher quality partautomatically instead of relying on a machine operator's adjustment ofthe process parameters after examining the quality of a finished part.Thus, the present invention provides real-time quality control for amanufacturing process. The importance of providing real-time control forthe machining operation increases with depth of the process. Forexample, in a boring process, the deeper the bore into the workpiece,the greater amount of chatter that will be expected during theoperation. Thus, without providing closed-loop feedback control for sucha process, the part quality would be expected to be low.

Although the present disclosure has focused mainly on turning machineoperations, the invention is not limited to any particular machiningoperation. The present invention could be applied to optimize millingoperations and other machine operations known to those skilled in theart. Also although the description has focused on the measurement of thedeflection and vibration of the tool, any measurement that providesinformation as to the performance of the tool in the cutting operationcan be used. Although the present disclosure has focused on closed-loopcontrol for CNC machines, the present invention is not so limited. Thepresent invention of measuring properties of a tool in real-time andadjusting the parameters of a machining operation in real-time based onthe measurement to provide higher quality parts can be applied to anymachining process. Although certain elements of a system have beendescribed as performing certain steps, the invention is not so limited.A particular element could perform several steps or the different stepscould be performed by several elements.

Although certain embodiments of the invention have been described, theinvention is not meant to be limited in any way to just theseembodiments. The invention is only meant to be limited by the appendedclaims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A closed-loop feedback control machiningsystem, the system comprising: a machine comprising: a workpiece holder;a tool holder; a spindle drive system for providing relative rotationbetween a workpiece and a tool; and a feed drive system for providingrelative translational movement between a workpiece and a tool; a tool;at least one sensor operatively associated with the tool; at least onesignal converter operatively associated with the at least one sensor; atleast one signal analyzer operatively associated with the at least onesignal converter; and at least one controller operatively associatedwith the at least one signal analyzer and operatively associated withthe spindle drive system and the feed drive system, wherein the at leastone sensor receives a signal from the tool and transmits the signal tothe at least one signal converter, wherein the at least one signalconverter converts the received signal to a set of digital parametersand transmits the digital parameters to the at least one signalanalyzer, wherein the at least one signal analyzer determines inreal-time at least one of the elements selected from the groupconsisting of: the force on the tool, the deflection of the tool, andthe vibration of the tool, and wherein the at least one controlleradjusts the power exerted by at least one of the spindle drive systemand the feed drive system in real-time based on the real-timedeterminations.
 2. The closed-loop feedback control machining system ofclaim 1, wherein the machine is a lathe.
 3. The closed-loop feedbackcontrol machining system of claim 1, wherein the tool is a boring bar.4. The closed-loop feedback control machining system of claim 1, whereinthe at least one sensor is selected from the group consisting of: fiberoptic sensors, magnetic sensors, and analog electronic strain gauges. 5.The closed-loop feedback control machining system of claim 2, whereinthe signal received from the tool is an optical signal.
 6. Theclosed-loop feedback control machining system of claim 1, wherein the atleast one controller is a CNC controller.
 7. The closed-loop feedbackcontrol machining system of claim 1, further comprising a portabledisplay device operatively associated with the at least one controllersuch that the portable display device can inform a user of theadjustments made by the controller.
 8. The closed-loop feedback controlmachining system of claim 7, wherein the portable display device isselected from the group consisting of a cellular phone, a PDA, and atablet PC.